Diesel engines (also referred to as compression ignition engines) produce an exhaust emission that generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM).
Oxidation catalysts comprising platinum group metals (PGMs) have been used to treat carbon monoxide (CO) and hydrocarbons (HCs), including the volatile organic fraction (VOF) of particulate matter (PM), in exhaust emissions produced by diesel engines. Such catalysts treat carbon monoxide (CO) by oxidising it to carbon dioxide (CO2), and treat hydrocarbons (HCs) by oxidising them to water (H2O) and carbon dioxide (CO2). Some platinum group metals, particularly when supported on a refractory oxide, can also promote the oxidation of nitric oxide (NO) to nitrogen dioxide (NO2).
As emissions standards for permissible emission of pollutants from diesel engines, particularly vehicular engines, become progressively tightened, there is a need to provide improved exhaust systems that are able to meet these standards and which are cost-effective. It is desirable to provide an oxidation catalyst that has excellent activity toward CO and HCs, and which is not readily poisoned by sulphur in diesel fuel. It is important that the oxidation catalyst works in conjunction with other emissions control devices, particularly as part of an exhaust system to maximise the overall reduction in pollutants produced by a diesel engine.
Platinum group metals (PGMs) are expensive metals. Catalyst manufacturers are under pressure to maximise the effectiveness of any PGMs that are included in an oxidation catalyst to minimise its material cost. The method of producing the oxidation catalyst is also important because there is a manufacturing cost associated with each step of applying a washcoat to a substrate and to the subsequent drying and calcination steps.
Our earlier WO 2007/077462 describes an exhaust system for a lean-burn internal combustion engine comprising a catalyst for oxidising carbon monoxide (CO) and hydrocarbons (HCs), where the catalyst comprises a first washcoat zone containing at least one PGM, which first washcoat zone being defined at an upstream end by an inlet end of a substrate monolith and at a downstream end by a point less than half way along a length of the substrate monolith measured from the inlet end; a second washcoat zone containing at least one PGM, which second washcoat zone comprising the point half way along the substrate monolith length measured from the inlet end; and a third washcoat zone containing at least one PGM, which third washcoat zone being defined at a downstream end by an outlet end of the substrate monolith and at an upstream end by a point at most three quarters of the way along the substrate monolith length from the inlet end, wherein both the PGM loading in the first washcoat zone and the PGM loading in the third washcoat zone is greater than the PGM loading in the second washcoat zone and wherein the first washcoat zone comprises a washcoat loading that is less than a washcoat loading of the third washcoat zone.